This invention relates to a method for manufacturing a bicycle chain drive. More particularly, this invention relates to a method for producing a novel bicycle crank and a novel crank and sprocket wheel assembly to form a chain drive which will provide a strong drive means while also being easier and less expensive to manufacture and assemble.
Typically, a sprocket wheel serves as the driving member in a chain drive for a bicycle. The sprocket wheel is normally driven by pedals through a crank made up of a pair of crank arms and an associated crank shaft connecting those arms. The crank is fastened to the sprocket on the right side of the bicycle and the crank shaft is suitably mounted in a bearing assembly supported in a housing incorporated as part of the bicycle frame.
Cranks have been fabricated in a number of different ways. Conventional methods of fabrication include the mounting of separate crank arms on a crank shaft, forming one crank arm as a part of the crank shaft with the other arm being detachably mounted to the shaft, drop forging the crank as a single piece, and cold forging as a single piece.
The cold forging process has been found advantageous in that it minimizes material scrap, provides close tolerances and results in higher strength while eliminating expensive secondary operations. In the past, cold formed cranks have been designed to drive the sprocket through a pin which is pressed into one of the crank arms, as shown in FIG. 4 of U.S. Pat. No. 3,608,184. This method of driving the sprocket, however, has been found to be less than optimum, for the pin has been found to be the weakest element of the sprocket drive. In addition the assembly of the sprocket wheel and crank is a three part operation since it involves the pressing of the pin into the appropriate crank arm prior to the mounting of the sprocket on the crank.
Other methods of driving the sprocket have been used in connection with cranks made up of a number of pieces and formed by other than the cold forging process. For example, U.S. Pat. No. 3,347,112 shows the use of a sprocket mounting segment which has a polygonal cross section. Such an arrangement is also suggested in U.S. Pat. No. 3,906,811 which describes mounting a sprocket by press fitting it onto a serrated collar which is part of a crank arm. Still another method shows a spline arrangement (U.S. Pat. No. 3,377,883). In the splined arrangement the splines are on a separate sleeve, which is in frictional engagement with the crank shaft.
The prior art methods described above all have disadvantages in that they do not minimize the number of parts to be assembled in manufacturing the crank and sprocket drive while at the same time providing a strong drive structure.
It is an object of this invention to provide a method for manufacturing a bicycle chain drive which is capable of producing a strong drive means for the chain while at the same time minimizing the number of parts which must be assembled and thus minimizing both the amount of labor involved in the manufacturing process and the cost of manufacture.
It is also an object of this invention to provide a method for manufacturing a bicycle crank having a minimum number of parts while also providing maximum strength and ease of manufacture.